The present invention relates to a cathode ray tube, and in particular to a cathode ray tube having a portion of a funnel portion for mounting a deflection yoke formed in generally truncated quadrilateral-pyramidal shape for the purpose of power saving and having a uniformity in thickness of an internal conductive coating improved in the portion of generally truncated quadrilateral-pyramidal shape to improve reliability of the cathode ray tube.
Generally, a cathode ray tube for displaying images or the like is provided with an evacuated envelope comprising a panel portion having a viewing screen formed of phosphor elements coated on an inner surface thereof, a neck portion for housing an electron gun and a funnel portion tapering down from a diameter of the panel portion to a diameter of the neck portion for connecting the panel portion and the neck portion.
In a color cathode ray tube for displaying color images, a color viewing screen is formed by coating phosphor elements of a plurality (usually three) of colors on the inner surface of the panel portion, a shadow mask serving as a color selection electrode is closely spaced from the screen and suspended within the panel portion, and an in-line type electron gun for emitting three electron beams is housed in the neck portion.
A stem provided at an end of the neck portion closes the end of the neck portion, supports the electron gun and supplies required voltages and a signal to the electron gun via respective stem pins embedded through the stem and arranged in a circular array, and a deflection yoke is mounted around the outside of the funnel portion for deflecting the electron beams two-dimensionally in the horizontal and vertical directions to reproduce an image on the screen.
Color cathode ray tubes for use in a high definition TV receiver or a monitor set for an information-processing terminal often employ higher voltages and a higher deflection frequency than before, and consequently increase deflection power consumption.
In such cathode ray tubes, as means for reducing power consumption of the deflection yoke, it is effective to reduce an outside diameter of a portion of the funnel portion around which the deflection yoke is mounted such that deflection magnetic fields act on the electron beams efficiently.
A portion of a funnel portion around which a deflection yoke is mounted will be hereinafter referred to merely as a yoke-mounting portion in this specification.
But, if an outside diameter of the yoke-mounting portion is reduced, some portions of the phosphor screen in the panel portion are sometimes not scanned by the electron beam because an inside diameter of the funnel portion in the vicinity of a junction of the funnel portion and the neck portion (that is, the smaller end of the funnel portion) becomes too small and the electron beam strikes the inner wall of the funnel portion when the electron beam is at maximum deflection.
In consideration of such facts, Japanese Patent Application Laid-open No. Hei 10-144238 (laid-open on May 29, 1998) discloses a cathode ray tube provided with a yoke-mounting portion of the funnel portion having an outer wall in the truncated quadrilateral-pyramidal shape so as to bring the deflection yoke in close proximity to electron beam and prevent occurrence of portions of the phosphor screen not scanned by the electron beam.
But, if the outer wall of the yoke-mounting portion of the funnel portion is formed in the truncated quadrilateral-pyramidal shape, the mechanical strength of the evacuated envelope is weakened and likelihood of implosion of the evacuated envelope becomes stronger. In Japanese Patent Application Laid-open No. Hei 10-144238, a reinforcing member is employed in a junction region of the panel portion and the yoke-mounting region of the truncated quadrilateral-pyramidal shape to prevent the implosion of the evacuated envelope. In a cathode ray tube disclosed therein, a cross-section of the outer wall of the yoke-mounting region perpendicular to its tube axis is a rectangle, and a cross-section of its inner wall is also a rectangle approximately similar to that of the outer wall of the yoke-mounting region.
For the purpose of eliminating the interception of an electron beam at the junction region (the smaller end of the funnel portion)of the funnel portion and the panel portion and preventing implosion of the evacuated envelope even when the maximum deflection of electron beam is increased, Japanese Utility Model Publication No. Sho 44-29152 (published on December 3) discloses a cathode ray tube in which a cross-section perpendicular to the tube axis, of an inner wall of the smaller end of the funnel portion is such that four sides or two parallel sides of the cross-section are curved convexly toward the tube axis at required regions thereof (that is, pincushion-shaped) and the corners of the cross-section are rounded.
In cathode ray tubes, the nearly entire area of the inner wall of the funnel portion and the inner wall of an area of the neck portion contiguous with the funnel portion are coated with an internal graphite film serving as an internal conductive coating.
For example, Japanese Patent Publication Nos. Sho 64-5741 (published on Jan. 31, 1989) and Hei 4-43374 (published on Jul. 16, 1992) and U.S. Pat. No. 4,403,170 (issued on Sep. 6, 1983) disclose techniques for dividing an internal graphite film in plural sections.
The internal graphite film forms a high-voltage filter capacitor with an external graphite film coated on the outer wall of the funnel portion, and conveys a high voltage applied to an anode button extending through a wall of a large-diameter portion of the funnel portion, to an anode electrode of an electron gun.
The internal graphite film is formed by coating a solution containing graphite particles, metallic oxides such as titanium oxide and iron oxide, water glass and others dispersed in a solvent on the inner wall of the funnel portion before the funnel portion is joined to the panel portion, and then drying.
Generally, the internal graphite film is coated by an automatic brushing machine in which the funnel portion is vertically oriented with its large-diameter end to be joined to the panel portion facing upward and its neck-portion side facing downward, and is rotated. As other techniques for coating, spraying and flow-coating are known.
In the above-mentioned cathode ray tube having a yoke-mounting portion formed in generally truncated quadrilateral-pyramidal shape, in the large-diameter region of the inner wall of the funnel portion there is no problem with coating of the internal graphite film because the inner wall in the large-diameter region of the funnel portion is nearly flat and consequently, non-uniformity in coating or formation of pools of coating material at corners of the quadrilateral-pyramidal shape does not occurs, but the yoke-mounting portion is prone to non-uniformity in coating of the graphite solution, formation of pools of the graphite solution at corners of the quadrilateral-pyramidal shape, or flowing down of the graphite solution into the neck portion, because the cross-sectional area of the inner wall is small in the yoke-mounting portion.
Specially, in the cathode ray tube in which the cross-section of the inner wall of the yoke-mounting portion is such that four sides or two parallel sides of the cross-section are curved convexly toward the tube axis at required regions thereof to form the shape of a pincushion, as disclosed in Japanese Utility Model Publication No. Sho 44-29152, a brush cannot contact the inner wall of the yoke-mounting portion closely, and consequently, non-uniformity in coating of the graphite solution, or formation of pools of the coating solution at corners of the quadrilateral-pyramidal shape occurs.
The objects of the above-mentioned first prior art related to the shape of the evacuated envelope are to eliminate occurrence of an area of the phosphor screen which is not scanned by an electron beam in a cathode ray tube of a wide deflection angle and to increase the mechanical strength of the evacuated envelope. The objects of the remaining prior art related to the internal graphite film are to eliminate an electrical discontinuity between the graphite film and an electrode and to reduce a spark current within the tube. None of the above prior art disclose or imply a problem with coating operation of the internal graphite film.
It is an object of the present invention to provide a cathode ray tube which facilitates and assures coating of the internal graphite on the inner wall of the yoke-mounting portion of its funnel portion.
To accomplish the above object, in accordance with an embodiment of the present invention, there is provided a cathode ray tube comprising: an evacuated envelope comprising a generally rectangular panel portion, a narrow neck portion having a circular cross-section and a funnel portion tapering down from a panel-portion side thereof toward a neck-portion side thereof for connecting said panel portion and said neck portion, said funnel portion being provided with a yoke-mounting portion of generally truncated quadrilateral-pyramidal shape for mounting a beam deflection yoke therearound on said neck-portion side of said funnel portion; a three-color phosphor screen formed on an inner surface of said panel portion; an electron gun housed in said neck portion; and an internal conductive film extending from an inner wall of said neck portion to an inner wall of said funnel portion, said internal conductive film comprising a first part and a second part, said first part being formed of graphite, metallic oxide and potassium silicate, said second part being formed of graphite and potassium silicate, wherein said first part extends from said neck portion in the vicinity of a forward end of said electron gun to a position in said yoke-mounting portion spaced a distance in a range of 60 mm to 150 mm from a splice line between said neck portion and said funnel portion, and said second part overlaps with said first part at opposing ends thereof and extends to a vicinity of a seal line between said funnel portion and said panel portion.
To accomplish the above object, in accordance with another embodiment of the present invention, there is provided a method of manufacturing a color cathode ray tube comprising an evacuated envelope comprising a generally rectangular panel portion, a narrow neck portion having a circular cross-section and a funnel portion tapering down from a panel-portion side thereof toward a neck-portion side thereof for connecting said panel portion and said neck portion, said funnel portion being provided with a yoke-mounting portion of generally truncated quadrilateral-pyramidal shape for mounting a beam deflection yoke therearound on said neck-portion side of said funnel portion, a three-color phosphor screen formed on an inner surface of said panel portion, an electron gun housed in said neck portion, and an internal conductive film extending from an inner wall of said neck portion to an inner wall of said funnel portion, comprising the steps of: forming a first part of said internal conductive film composed of graphite, metallic oxide and potassium silicate in an area extending from said neck portion in the vicinity of a forward end of said electron gun to a position in said yoke-mounting portion spaced a distance in a range of 60 mm to 150 mm from a splice line between said neck portion and said funnel portion, and forming a second part of said internal conductive film composed of graphite and potassium silicate in an area overlapping with an end of said first part on a panel-portion side thereof and extending to a vicinity of a seal line between said funnel portion and said panel portion.
To accomplish the above object, in accordance with still another embodiment of the present invention, there is provided a method of manufacturing a color cathode ray tube comprising an evacuated envelope comprising a generally rectangular panel portion, a narrow neck portion having a circular cross-section and a funnel portion tapering down from a panel-portion side thereof toward a neck-portion side thereof for connecting said panel portion and said neck portion, said funnel portion being provided with a yoke-mounting portion of generally truncated quadrilateral-pyramidal shape for mounting a beam deflection yoke therearound on said neck-portion side of said funnel portion, a three-color phosphor screen formed on an inner surface of said panel portion, an electron gun housed in said neck portion, and an internal conductive film extending from an inner wall of said neck portion to an inner wall of said funnel portion, comprising the steps of: forming an upstream subpart of a first part of said internal conductive film composed principally of
15 to 25 weight % graphite,
15 to 25 weight % titanium oxide and
45 to 70 weight % potassium silicate in an area extending from said neck portion in the vicinity of a forward end of said electron gun to a first position in the vicinity of a splice line between said neck portion and said funnel portion; forming a downstream subpart of said first part of said internal conductive film composed principally of
15 to 25 weight % graphite,
45 to 70 weight % titanium oxide and
35 to 60 weight % potassium silicate in an area contiguous to a forward boundary of said upstream subpart and extending to a second position in said yoke-mounting portion spaced a distance in a range of 60 mm to 150 mm from said splice line; and forming a second part of said internal conductive film composed principally of
60 to 70 weight % graphite and
35 to 45 weight % potassium silicate in an area overlapping with an end of said downstream subpart of said first part on a panel-portion side thereof and extending to a vicinity of a seal line between said funnel portion and said panel portion.
With the configuration of the present invention, a highly reliable cathode ray tube with its deflection power consumption reduced and a method of making the same are obtained wherein the operation of coating of the internal graphite film on the yoke-mounting portion is facilitated such that non-uniformity in thickness caused by non-uniform coating of the graphite solution and by formation of pools of the graphite solution at corners of the quadrilateral-pyramidal shape, or flowing down of the graphite solution into the neck portion is prevented.